This invention relates to a method and apparatus for detecting impending saturation in magnetic materials, such as, but not limited to, the core of power transformers in switched-mode power converters.
The undesirable saturation of electronic and magnetic devices has always been troublesome to circuit designers, because the boundaries of saturation impose limitations on the performance of almost any design. Saturation of most electronic devices is generally easy to avoid, because the physical quantities which are responsible for the saturation (voltages or currents) are nearly always quite accurately known and are easily controlled. The problem of magnetic saturation, on the other hand, has been much more difficult to overcome, owing to the lack of good practical way to measure the quantity which saturates the magnetic material, namely the flux density. The importance of finding a solution to this problem has become more acute with the advent of switched-mode power conversion because unexpected saturation of a high-frequency power transformer not only degrades the performance of the converter, but often induces spectacular catastrophic failures.
In order to take full advantage of available magnetic material and simultaneously meet minimum size and weight constraints, an isolation transformer of a dc-to-dc power converter is required to be designed to handle only ac currents. For a compact, efficient, and otherwise high-performance design, no net dc magnetization current can be permitted in the transformer, as that would cause the material to saturate.
Whole or partial saturation of the power transformer is undesirable for many reasons, but the most significant one is that it causes excessive and often destructive stresses on the power semiconductor devices in the converter. Unfortunately, unless one takes definite measures to prevent it, it is possible for a well-designed transformer to saturate as a result of transient excitations or naturally-occurring nonidealities of real devices. The saturation is caused by a dc magnetization current which builds as the result of a momentary volt-second imbalance on the transformer over one or more switching periods.